The present invention relates to a composition for use in a transparent and electrically conductive film and a method for making the film and more particularly to a composition for a transparent and electrically conductive film formed on a glass substrate or a ceramic substrate and a method for making the film.
Hitherto, an electrode material having a high transmittance for a visible light has been used in a display element such as a liquid crystal display and an electroluminesence display and in a heating resistor for preventing the window glass of a car,an air plane and building from collecting moisture or from freezing.
There has been known a transparent and electrically conductive material usually consisted of antimony tin oxide(ATO) or indium tin oxide(ITO). These metal oxides can be applied to the glass substrate or a ceramic substrate for formation of transparent and electrically conductive film.
The transparent and electrically conductive film can be formed by the following methods: (1) vacuum deposition method; (2) sputtering method; (3) CVD method; (4) screen printing method and (5) dipping method.
However, the above methods (1), (2) and (3) require a complicated equipment in a high cost and have a problem in the cost and the mass production. The methods (4) and (5) have a possibility to solve the problem due to the methods (1), (2) and (3), but has a difficulty to form a film superior in the practical use.
For example, a use of organic solution of an inorganic compound such as indium nitrate, indium chloride or stannic chloride causes the formed film to generate a white spot or to have a weak mechanical strength which results in an easy scratching. There is a method to use an indium organic acid compound having a strong ion bonding such as indium octylate. Since the organic indium compound tends to be hydrolized and is relatively easy to be chemically changed, it is disadvantageous that the application method of liquid generates a gel state. Further, there is proposed another method to use an organic complex of indium or tin. In this method, an evaporation of tin compound during the heat treatment accompanied with thermal decomposition of the film applied to the glass substrate prevents the resultant film from forming an uniform structure. It is difficult for this method to obtain a uniform film having a low electric resistance.
In order to solve the problem mentioned above, an object of the present invention is to provide a composition capable of forming a transparent and electrically conductive film having a low electric resistance and a high transmittance without the evaporation of tin compound during the heat treatment and a method for making the film by use of the composition.
As a result of our sharp study, we found that if a composition to be applied on a substrate contains a Tin compound in a complex form represented by the formula:InX.sub.p-r SnY.sub.q-s Zr.sub.s+, the tin compound is not easy to evaporate during the heat treatment. Therefore, in a first aspect in order to achieve the object, the present invention is to provide a composition for use in a formation of a transparent and electrically conductive film, which essentially comprises a composite compound represented by the formula: EQU InX.sub.p-r SnY.sub.q-s Z.sub.r+s
wherein InX.sub.p is an inorganic indium salt, SnY.sub.q is an organic tin salt and Z is a organic compound capable of coordinating with indium and tin; p, q, r and s are coordination numbers.
In accordance with a composition for use in a formation of transparent and electrically conductive film, a mixture of inorganic indium compound and organic tin compound is combined with organic compound capable of coordinating with both of indium and tin to form an organic solution. When the organic solution is heated, the inorganic indium compound and the organic tin compound coordinating partially with the organic compound react with water of crystallization of inorganic indium compound. The organic tin compound is partially hydrolyzed to form the intermediate complex compound including indium and tin as represented by the above formula. This suppresses the evaporation of tin or tin compound and provides a uniform film having a high transmittance and a low electric resistance.
Accordingly, in a further aspect of the present invention, there is provided a composition for use in a formation of a transparent and electrically conductive film , which is prepared by partial hydrolysis under heat treatment of an organic solution mainly containing an inorganic indium compound, an organic tin compound and an organic compound capable of coordinating with indium and tin.
In the organic solution, said organic tin salt is preferablly mixed with said inorganic indium salt in a mixing ratio of 5 to 20 wt % by a definition of Sn/(Sn+In) .times. 100%.
Further, it is preferred that said inorganic indium salt is selected from the group consisting of indium nitrate, indium sulfate and indium chloride. Also, it is preferred that said organic tin salt is selected from the group consisting of salts of a carboxylic acid and a dicarboxylic acid. Furthermore, it is preferred that said organic compound capable of coordinating with tin and indium is selected from the group consisting of .beta.-diketone group, .alpha. or .beta.-ketone acid group, ester group of .alpha. or .beta.-ketone acid group and .alpha. or .beta.-amino alcohol group.
An addition of polyhydric alcohol increases the viscosity of the organic solution and gives a high stability to a resultant film obtained by applying the solution to the substrate and firing or firing. Accordingly, in a third aspect of the present invention, there is provided a composition for use in a formation of a transparent and electrically conductive film which further contains a polyhydric alcohol.
Since the composition for use in a formation of a transparent and electrically conductive film is fixed on the substrate, it is preferred that the composition further contains a catalyzer for thermal decomposition in order to promote a thermal decomposition. The catalyzer is preferablly selected from the group consisting of a peroxide compound and a nitro compound. Preferred examples are hydrogen peroxide, tri-nitrotoluene and picric acid since these compounds having small carbon number tend to leave little residual carbon after the thermal decomposition thereof.
In a fourth aspect of the present invention, there is provived a method for making a transparent and electrically conductive film, which comprises steps of providing a composition for use in a formation of a transparent and electrically conductive film by heating an organic solution mainly containing an inorganic indium salt, an organic tin salt and organic compound capable of coordinating with indium and tin to carry out partial hydrolysis of said organic solution and if necessary, thereafter adding a polyhydric alcohol and/or a catalyzer for thermal decomposition to a resultant organic solution; applying said composition to a substrate and after drying subjecting the substrate having said composition film applied thereto to a firing treatment.
In the method for making a transparent and electrically conductive film, it is preferred that the firing treatment is carried out in a temperature rising speed of 20 .degree. C./min. or more and under a pressurized atmosphere of steam in order to promote a hydrolysis reaction.
In order to carry out the method for making a transparent and electrically conductive film under a reducing atmosphere, it is necessary that the organic tin salt has an oxygen content of 22 at % or more. Even if the firing treatment is carried out under the reducing atmosphere, the organic compound capable of coordinating with both of indium and tin can evaporate before the thermal decomposition thereof and the inorganic indium compound and the organic tin compound can thermally decompose by means of their own oxygen, so that the oxygen defect amount becomes to be increased and thus the resulting film resistance can be lowered, comparing with that of a film formed by firing under a conventional oxygen atmosphere.
In the method for making a transparent and electrically conductive film in order to achieve a uniform heating the composition applied on the substrate to prevent a formation of semi-hardened or cracked skin layer on a surface of the film, it is preferred that the firing treatment is carried out by means of thermal decomposition due to a radiant heat under a semi-closed atmosphere. Such a thermal decomposition under the semi-closed atmosphere may be carried out in a room defined by walls made of metal, glass or ceramics material provided with small holes, whereby the atmosphere pressure can be self-controlled by means of a ratio B/A between (B) an amount of decomposed gas generating on the thermal decomposition of the composition applied on the substrate and the (A) total area of the small holes for discharging the decomposed gas.
In an embodiment for carrying out the above method for making a transparent and electrically conductive film, an oven provided with the following design conditions can realize the best mode of the thermal decomposition:
(1) wherein the thermal decomposition under the semi-closed atmosphere is carried out in a room defined by walls, each of which temperature can be controlled independently in a manner to make a property of transparent and electrically conductive film formed on front and back surfaces of the substrate to be uniform. PA1 (2) wherein the thermal decomposition under the semi-closed atmosphere is carried out by a direct heat from the walls or by a indirect heat in a semi-closed box heated by a receiving heat from the walls. PA1 (3) wherein the pressure of the semi-closed atmosphere is from 5 to 100mmH.sub.2 O when the thermal decomposition is going. PA1 (4) wherein a ratio (V/A) between the room volume (V) of the semi-closed atmosphere and the total area (A) of the holes for discharging the decomposed gas is from 50 to 2000 cm.sup.3 /cm.sup.2. PA1 (5) wherein a ratio (B/A) between the amount (B) of the decomposed gas and the total area (A) of the holes for discharging the decomposed gas is from 100 to 10000 ml/cm.sup.3. PA1 (6) wherein a ratio (B/V) between the amount (B) of the decomposed gas and the room volume (V) of the semi-closed atmosphere is from 0.01 to 1000 ml/cm.sup.3.
Meanwhile, according to the method of the present invention, there are provided various kinds of the transparent and electrically conductive films. Among them, as a result of our studying a relation between the average particle size and the electrical resistance or the visible light transmittance (at 550 nm), we found that the films which comprise oxide particles of indium and tin having a particle diameter of 30 to 100 nm are superior in the electrical resistance as shown in FIG. 2, since a particle size of 100 nm or more makes a contact resistance between the particles to be large and thus results in a lower packing density. On the contrary, a particle size of 30 nm or less makes a film strength to be weak and thus makes a film resistance to be large.
Also, we found that the more uniform the avarage particle size becomes, the higher strength the film is provided with. Accordingly, the present invention is to provide a transparent and electrically conductive film, wherein 90% of the oxide particles is within a distribution of center particle size .+-. 3.sigma., whereby the film comes to have a much lower resistance.
Further, as a result of studying the relation between the average particle size and the film thickness, we found that it is preferable that a ratio a/b between the average particle size (a) and the film thickness (b) is 0.7 or less, because a number of particles arranged in a series to a thickness direction of the film has a much influence on the film strength.